The present invention relates to a manufacturing method of a rotor for a permanent magnet-excited, high-speed electric rotary machine, particularly to a manufacturing method of a rotor for a large-capacity permanent magnet-excited, high-speed electric rotary machine in which the rotor is rotated at a high speed, for example, not less than thousands of revolutions per minute. The present invention also relates to the rotor and to the electric rotary machine including the rotor.
In general, a rotor for a permanent magnet-excited electric rotary machine includes a rotor shaft, a sleeve and cylindrical permanent magnet assembly which is closely fitted on the rotor shaft, and a protection cover fitted on a cylindrical permanent magnet of the assembly, as disclosed in Japanese Patent Unexamined Publication No. 6-284611. British Patent Application No. GB2299217A and Japanese Patent Unexamined Publication No. 5-22880 each discloses a rotor which includes a plurality of cylindrical permanent magnets and a rotor shaft inserted in the cylindrical permanent magnets. Japanese Patent Unexamined Publication No. 9-19093 discloses a manufacturing method of a protection cover of a fiber-reinforced plastic. Japanese Patent Unexamined Publication No. 8-107641 discloses a protection cover made of a fiber-reinforced plastic. U.S. Pat. No. 5,485,045 discloses a method for attaching a protection cover to a permanent magnet by utilizing a taper form.
In order to rotate a rotor at a high speed, it must be made so that slip is not occurred between the rotor shaft and a permanent magnet assembly by the centrifugal force due to the high-speed rotation. For this purpose, a large interference is needed between the rotor shaft and permanent magnet assembly.
For fitting a permanent magnet on the rotor shaft, the usual shrink fit cannot be applied. This is because the permanent magnet is not largely expanded by being heated because of its small coefficient of thermal expansion, so that the large interference cannot be obtained.
For obtaining an interference between the cylindrical permanent magnet and the rotor shaft, a method is known in which the magnet and a protection cover is fitted on the cooled rotor shaft because the coefficients of thermal expansion of a fiber-reinforced plastic of which the protection cover is made and the magnet material are almost zero. In this method, the interference obtained is determined by the temperature of the rotor shaft. For the most proper method for lowering the temperature of the rotor shaft, there is a method using liquid nitrogen. However, even by this method, the temperature difference obtained is only to the extent of about 170.degree. C. to about 180.degree. C. The higher the rotor speed is, the smaller the diameter of the rotor shaft to be employed is and therefore, an amount of thermal contraction becomes small accordingly. It is therefore difficult to obtain the necessary interference for the high-speed rotor by this method when machining tolerance, working margin and so on are taken into consideration.
Besides, when the cooled rotor shaft comes back gradually to the normal temperature, the rotor shaft enlarges also in an axial direction thereof. However, because the magnet and the protection cover made of the fiber-reinforced plastic hardly enlarge, the protection cover is stretched in the axial direction that the strength thereof is smaller. This may cause that the protection cover is broken. It is thus difficult to apply such a cooling fitting or expansion fitting in assembling the high-speed rotor including the fiber-reinforced plastic (FRP) protection cover.
On the other hand, to increase the capacity of the rotor, a rare-earth sintering magnet having a high flux density must be used for the cylindrical permanent magnet. In such a rare-earth sintered magnet, however, the degree of freedom in shaping is low. In particular, in a cylindrical magnet, it is not possible to enlarge the length of the cylindrical magnet in contrast to an outer diameter thereof. For this reason, in case of large-capacity rotor, a plurality of cylindrical magnets must be juxtaposed on the sleeve in the axial direction. As a result, the total length in the axial direction of the rotor becomes long. Therefore, the stroke for press fitting the rotor shaft into the sleeve becomes long and upon press fitting of the rotor shaft, buckling and/or breakage of the permanent magnets and/or the protection cover occur. Thus, mechanical press fitting cannot be applicable.